Gravity Field and Steady-State Ocean Circulation Explorer
Encyclopedia
The Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) is an ESA satellite
that was launched on March 17, 2009. It is a satellite carrying a highly sensitive gravity gradiometer
which detects fine density differences in the crust and oceans of the Earth.
GOCE data will have many uses, probing hazardous volcanic regions and bringing new insight into ocean behaviour. The latter, in particular, is a major driver for the mission. By combining the gravity data with information about sea surface height
gathered by other satellite altimeters, scientists will be able to track the direction and speed of geostrophic ocean currents. The low orbit and high accuracy of the system will greatly improve the known accuracy and spatial resolution of the geoid
(the theoretical surface of equal gravitational potential on the Earth).
The satellite's arrow shape and fins help keep the GOCE stable as it flies through the wisps of air still present at an altitude of 260 km. In addition, an ion propulsion system continuously compensates for the deceleration of air-drag without the vibration of a conventional chemically-powered rocket engine
, thus restoring the path of the craft as closely as possible to a purely inertial trajectory. The craft's primary instrument is three pairs of highly sensitive accelerometer
s which measure gravitational gradients along three different axes.
To increase resolution, the satellite will fly in an unusually low orbit.
(AGU) 2010 Fall Meeting by Dr Rory Bingham from Newcastle University, UK.
The maps produced from the GOCE data show ocean currents in much finer detail than previously available.
Even very small details like the Mann Eddy
in the North Atlantic are visible in the data, as was the effect of Hurricane Igor (2010)
.
in northern Russia with a Rockot
vehicle at 15:21 CET (14:21 UT). The Rockot is a modified SS-19 intercontinental ballistic missile that was decommissioned after the Strategic Arms Reduction Treaty. The launcher uses the two lower liquid fuel stages of the original SS-19 and is equipped with a Briz-KM third stage developed for precise orbit injection. GOCE was launched into a Sun-synchronous
dusk-dawn orbit with an inclination of 96.70° and an ascending node at 18:00. Separation from the launcher was at 295 km. The satellite’s orbit will then decay over a period of 45 days to an operational altitude, currently planned at 270 km. During this time, the spacecraft will be commissioned and the electrical propulsion system will be checked for reliability in altitude control.
The first launch attempt on 16 March 2009, was aborted due to a malfunction with the launch tower. Liftoff occurred successfully at 14:21 GMT on 17 March 2009. The Rockot launcher delivered the satellite northward over the Arctic. About 90 minutes later, after one orbital revolution and two Briz-KM upper-stage burns, the spacecraft was successfully released into a circumpolar orbit at 280 km altitude with 96.7° inclination to the Equator. Soon after the separation, contact was successfully established with the satellite.
In February 2010 a fault was discovered in the satellite's computer, which meant controllers were forced to switch control to the backup computer. In July 2010, GOCE suffered a serious communications malfunction, when the satellite suddenly failed to downlink scientific data to its receiving stations. Extensive investigations by experts from ESA and industry revealed that the issue was almost certainly related to a communication link between the processor module and the telemetry modules of the main computer.
The recovery was completed in September 2010: as part of the action plan, the temperature of the floor hosting the computers was raised by some 7°C – resulting in restoration of normal communications.
The first Earth global gravity model based on GOCE data was presented at ESA’s Living Planet Symposium, in June 2010.
In November, 2010, it was decided to extend the mission lifetime of 18 months, till the end of 2012, in order to improve the collected data.
. They are arranged in three pairs of ultra-sensitive accelerometers arranged in three dimensions that respond to tiny variations in the 'gravitational tug' of the Earth
as it travels along its orbital path. Because of their different position in the gravitational field they all experience the gravitational acceleration of the Earth slightly differently. The three axes of the gradiometer
allow the simultaneous measurement of the five independent components of the gravity gradient tensor
.
Other payload is an onboard GPS receiver used as a Satellite-to-Satellite Tracking Instrument (SSTI); a compensation system for all non-gravitational forces acting on the spacecraft. The satellite is also equipped with a laser
retroreflector
to enable tracking by ground-based lasers.
, which will produce 1,300 W
of power and cover the Sun-facing side of GOCE.
The ion propulsion electric engine ejects xenon
ions at velocities exceeding 40,000 m/s, which will compensate for the orbital decay losses. GOCE's mission will end when the 40 kg xenon fuel tank empties (with a predicted lifetime of about 20 months). However, the ESA has reported that unusually low solar activity (meaning a calmer upper atmosphere, and hence less drag on the craft) may mean the mission could extend past its predicted 20 months due to fuel savings - possibly into 2014.
The frame incorporates the fixed solar panels as fins to stabilise the spacecraft while it orbits through the residual air in the ionosphere
(also called thermosphere
).
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
that was launched on March 17, 2009. It is a satellite carrying a highly sensitive gravity gradiometer
Gradiometer
A gradiometer measures the gradient of a physical quantity, such as a magnetic field or gravity.-Types of gradiometer:There are at least two types of gradiometer to measure magnetic fields:...
which detects fine density differences in the crust and oceans of the Earth.
GOCE data will have many uses, probing hazardous volcanic regions and bringing new insight into ocean behaviour. The latter, in particular, is a major driver for the mission. By combining the gravity data with information about sea surface height
Sea surface height
Sea surface height is the height of the ocean's surface. On a daily basis, SSH is most obviously affected by the tidal forces of the Moon and the Sun acting on the Earth. Over longer timescales, SSH is influenced by ocean circulation. Typically, SSH anomalies resulting from these forces differ...
gathered by other satellite altimeters, scientists will be able to track the direction and speed of geostrophic ocean currents. The low orbit and high accuracy of the system will greatly improve the known accuracy and spatial resolution of the geoid
Geoid
The geoid is that equipotential surface which would coincide exactly with the mean ocean surface of the Earth, if the oceans were in equilibrium, at rest , and extended through the continents . According to C.F...
(the theoretical surface of equal gravitational potential on the Earth).
The satellite's arrow shape and fins help keep the GOCE stable as it flies through the wisps of air still present at an altitude of 260 km. In addition, an ion propulsion system continuously compensates for the deceleration of air-drag without the vibration of a conventional chemically-powered rocket engine
Rocket engine
A rocket engine, or simply "rocket", is a jet engineRocket Propulsion Elements; 7th edition- chapter 1 that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...
, thus restoring the path of the craft as closely as possible to a purely inertial trajectory. The craft's primary instrument is three pairs of highly sensitive accelerometer
Accelerometer
An accelerometer is a device that measures proper acceleration, also called the four-acceleration. This is not necessarily the same as the coordinate acceleration , but is rather the type of acceleration associated with the phenomenon of weight experienced by a test mass that resides in the frame...
s which measure gravitational gradients along three different axes.
Mission objectives
- To determine gravity-field anomalies with an accuracy of (1 mGalGal (unit)The gal, sometimes called galileo, is a unit of acceleration used extensively in the science of gravimetry. The gal is defined as 1 centimeter per second squared ....
).
To increase resolution, the satellite will fly in an unusually low orbit.
- To determine the geoidGeoidThe geoid is that equipotential surface which would coincide exactly with the mean ocean surface of the Earth, if the oceans were in equilibrium, at rest , and extended through the continents . According to C.F...
with an accuracy of 1–2 cm. - To achieve the above at a spatial resolution better than 100 km.
Discoveries and applications
The final gravity map and model of the geoid will provide users worldwide with well-defined data product that will lead to:- A better understanding of the physics of the Earth's interior to gain new insights into the geodynamics associated with the lithosphere, mantle composition and rheology, uplift and subduction processes.
- A better understanding of the ocean currents and heat transport.
- A global height-reference system, which can serve as a reference surface for the study of topographic processes and sea-level change.
- Better estimates of the thickness of polar ice-sheets and their movement.
Initial findings
Initial results of the GOCE satellite mission were presented at the American Geophysical UnionAmerican Geophysical Union
The American Geophysical Union is a nonprofit organization of geophysicists, consisting of over 50,000 members from over 135 countries. AGU's activities are focused on the organization and dissemination of scientific information in the interdisciplinary and international field of geophysics...
(AGU) 2010 Fall Meeting by Dr Rory Bingham from Newcastle University, UK.
The maps produced from the GOCE data show ocean currents in much finer detail than previously available.
Even very small details like the Mann Eddy
Mann Eddy
The Mann Eddy is a very small feature of ocean currents in the Atlantic. It is a persistent clockwise circulation in the middle of the North Atlantic ocean...
in the North Atlantic are visible in the data, as was the effect of Hurricane Igor (2010)
Hurricane Igor (2010)
Hurricane Igor was the most destructive tropical cyclone to strike the Canadian island of Newfoundland on record. The origins of Igor were within a broad area of low pressure that moved off the west coast of Africa on September 6, 2010. Tracking slowly westward, it developed into a tropical...
.
Launch and operations
GOCE was launched from the Plesetsk CosmodromePlesetsk Cosmodrome
Plesetsk Cosmodrome is a Russian spaceport, located in Arkhangelsk Oblast, about 800 km north of Moscow and approximately 200 km south of Arkhangelsk.-Overview:...
in northern Russia with a Rockot
Rockot
The Rokot , also transliterated as a the pun Rockot, is a Russian space launch vehicle that can launch a payload of 1,950 kilograms into a 200 kilometre high Earth orbit with 63° inclination. It is a derivative of the UR-100N intercontinental ballistic missile , supplied and operated by Eurockot...
vehicle at 15:21 CET (14:21 UT). The Rockot is a modified SS-19 intercontinental ballistic missile that was decommissioned after the Strategic Arms Reduction Treaty. The launcher uses the two lower liquid fuel stages of the original SS-19 and is equipped with a Briz-KM third stage developed for precise orbit injection. GOCE was launched into a Sun-synchronous
Sun-synchronous orbit
A Sun-synchronous orbit is a geocentric orbit which combines altitude and inclination in such a way that an object on that orbit ascends or descends over any given point of the Earth's surface at the same local mean solar time. The surface illumination angle will be nearly the same every time...
dusk-dawn orbit with an inclination of 96.70° and an ascending node at 18:00. Separation from the launcher was at 295 km. The satellite’s orbit will then decay over a period of 45 days to an operational altitude, currently planned at 270 km. During this time, the spacecraft will be commissioned and the electrical propulsion system will be checked for reliability in altitude control.
The first launch attempt on 16 March 2009, was aborted due to a malfunction with the launch tower. Liftoff occurred successfully at 14:21 GMT on 17 March 2009. The Rockot launcher delivered the satellite northward over the Arctic. About 90 minutes later, after one orbital revolution and two Briz-KM upper-stage burns, the spacecraft was successfully released into a circumpolar orbit at 280 km altitude with 96.7° inclination to the Equator. Soon after the separation, contact was successfully established with the satellite.
In February 2010 a fault was discovered in the satellite's computer, which meant controllers were forced to switch control to the backup computer. In July 2010, GOCE suffered a serious communications malfunction, when the satellite suddenly failed to downlink scientific data to its receiving stations. Extensive investigations by experts from ESA and industry revealed that the issue was almost certainly related to a communication link between the processor module and the telemetry modules of the main computer.
The recovery was completed in September 2010: as part of the action plan, the temperature of the floor hosting the computers was raised by some 7°C – resulting in restoration of normal communications.
The first Earth global gravity model based on GOCE data was presented at ESA’s Living Planet Symposium, in June 2010.
In November, 2010, it was decided to extend the mission lifetime of 18 months, till the end of 2012, in order to improve the collected data.
Payload
The satellite's main payload is the Electrostatic Gravity Gradiometer (EGG) to measure the gravity field of EarthPhysical geodesy
Physical geodesy is the study of the physical properties of the gravity field of the Earth, the geopotential, with a view to their application in geodesy.-Measurement procedure:...
. They are arranged in three pairs of ultra-sensitive accelerometers arranged in three dimensions that respond to tiny variations in the 'gravitational tug' of the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
as it travels along its orbital path. Because of their different position in the gravitational field they all experience the gravitational acceleration of the Earth slightly differently. The three axes of the gradiometer
Gravity Gradiometry
Gravity gradiometry is the study and measurement of variations in theacceleration due to gravity. The gravity gradient is the spatial rate of change of gravitational acceleration....
allow the simultaneous measurement of the five independent components of the gravity gradient tensor
Tensor
Tensors are geometric objects that describe linear relations between vectors, scalars, and other tensors. Elementary examples include the dot product, the cross product, and linear maps. Vectors and scalars themselves are also tensors. A tensor can be represented as a multi-dimensional array of...
.
Other payload is an onboard GPS receiver used as a Satellite-to-Satellite Tracking Instrument (SSTI); a compensation system for all non-gravitational forces acting on the spacecraft. The satellite is also equipped with a laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
retroreflector
Retroreflector
A retroreflector is a device or surface that reflects light back to its source with a minimum scattering of light. An electromagnetic wave front is reflected back along a vector that is parallel to but opposite in direction from the wave's source. The device or surface's angle of incidence is...
to enable tracking by ground-based lasers.
Power
GOCE has fixed solar panelsPhotovoltaic system
A photovoltaic system is a system which uses one or more solar panels to convert sunlight into electricity. It consists of multiple components, including the photovoltaic modules, mechanical and electrical connections and mountings and means of regulating and/or modifying the electrical...
, which will produce 1,300 W
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...
of power and cover the Sun-facing side of GOCE.
The ion propulsion electric engine ejects xenon
Xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...
ions at velocities exceeding 40,000 m/s, which will compensate for the orbital decay losses. GOCE's mission will end when the 40 kg xenon fuel tank empties (with a predicted lifetime of about 20 months). However, the ESA has reported that unusually low solar activity (meaning a calmer upper atmosphere, and hence less drag on the craft) may mean the mission could extend past its predicted 20 months due to fuel savings - possibly into 2014.
The frame incorporates the fixed solar panels as fins to stabilise the spacecraft while it orbits through the residual air in the ionosphere
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
(also called thermosphere
Thermosphere
The thermosphere is the biggest of all the layers of the Earth's atmosphere directly above the mesosphere and directly below the exosphere. Within this layer, ultraviolet radiation causes ionization. The International Space Station has a stable orbit within the middle of the thermosphere, between...
).
See also
- CryoSat-2CryoSat-2CryoSat-2 is a European Space Agency environmental research satellite which was launched in April 2010. It provides scientists with data about the polar ice caps and tracks changes in the thickness of the ice with a resolution of about . This information is useful for monitoring climate change...
ESA - launch 2010 - Gravity Recovery and Climate ExperimentGravity Recovery and Climate ExperimentThe Gravity Recovery And Climate Experiment , a joint mission of NASA and the German Space Agency, has been making detailed measurements of Earth's gravity field since its launch in March 2002....
(GRACE) NASA - launch 2002 - ICESatICESatICESat , part of NASA's Earth Observing System, was a satellite mission for measuring ice sheet mass balance, cloud and aerosol heights, as well as land topography and vegetation characteristics...
, (Ice, Cloud, and land Elevation Satellite) NASA - launch 2003 - List of climate research satellites
- Soil Moisture and Ocean Salinity satelliteSoil Moisture and Ocean Salinity satelliteThe Soil Moisture and Ocean Salinity Satellite is a part of ESA's Living Planet Programme intended to provide new insights into Earth's water cycle and climate...
(SMOS) ESA - launch 2009
External links
- ESA GOCE site
- ESA's gravity mission GOCE
- GOCE page at ESA Spacecraft Operations
- GOCE Mission Profile by NASA's Solar System Exploration
- 24 October 2008 BBC News story
- 6 April 2009 BBC News story
- Goce satellite views Earth's gravity in high definition BBC News (includes a copy of the new gravity map) 29 June 2010
- earthzine.org